Concentrated sulfuric acid dissolves narcotine to a colorless liquid. If the acid contains a trace of nitric acid, as happens often with the ordinary chemically pure acid, the solution turns yellow on standing and red by warming. The red is easily obtained by dissolving in a little dilute acid, then evaporating from a test-tube very slowly by means of a small flame. After the red liquid cools a trace of nitric acid changes it to violet.

N1. Concentrated nitric acid dissolves narcotine to a yellow solution.

N2. Fröhde's reagent of the ordinary strength dissolves narcotine, forming a green solution.  If stronger (O.01 g molybdate in 1 c.c. of acid) the green solution soon changes to cherry red.

N3. Erdmann's reagent gives a red solution.
 
N4. Vanadium sulfate gives a red color likewise.